200 



may be joined together mid-ventrally. The arches hear 

 gill-rays (of independent origin). 



The foremost of the arches is the mandibular, the second 

 is the hyoid, and the others are called branchial arches 

 (1st, 2nd, etc.) because they support the gills. In all 

 Craniates, except Cyclostomata, the jaws which support 

 the mouth are formed from the mandibular arch, each half 

 of which consists of an upper portion, the palato-quadrate, 

 with a forward palato-pterygoid process, and a lower portion 

 known as Meckel's cartilage. In Dipnoi and Amphibians, 

 the palato-pterygoid is greatly reduced, and the palato- 

 quadrate becomes the upper jaw and is fused with the 

 cranium. Meckel's cartilage is segmented off to form the 

 lower jaw, which is connected with the cranium by the 

 dorsal portion of its own (mandibular) arch ; and therefore 

 the skull is described as autostylic or protostylic. In Elasmo- 

 branch fishes the palato-quadrate is greatly reduced and 

 the palato-pterygoid becomes the upper jaw, but it is 

 suspended by ligaments to the cranium, and, along with 

 Meckel's cartilage (the lower jaw), is connected with the 

 cranium by the hyomandibular, the upper portion of the 

 hyoid arch ; therefore the Elasmobranch skull is hyostylic. 

 When the connection is partly autostylic and partly hyo- 

 stylic, the skull is described as amphistylic (e.g., Teleosteans). 

 As already noted, the upper portion of the hyoid arch 

 is the hyomandibular ; the lower portion being the hyoid 

 proper. In Fishes the hyoid and branchial arches are 

 concerned with respiration and are well-developed ; in 

 the air-breathing Craniates these arches are by atrophy 

 reduced to remnants which are fused together to form a 

 supporting skeleton in connection with the tongue (e.g,. 

 hyoid plate of frog, and hyoid of mammals). 



State the Origin of the Central Nervous System.. 



The central nervous system arises as a thickening of the 

 ectoderm called the medullary plate, which later becomes 

 a medullary groove. The sides of the groove, the medullary 

 folds, coalesce dorsally, and thus the neural tube is formed ; 

 it is separated off from the ectoderm, and is surrounded 

 by mesenchyme. The greater part of the tube becomes the 

 spinal cord ; the brain is developed from the anterior 

 dilated portion or primary vesicle. A detached portion of 

 the central nervous system is formed as a neural crest 

 above the neural tube ; it is divided into two longitudinal 

 halves which develop outgrowths, the ganglia, and from 



